Evaluating Integrated Weed Management Tactics to Reduce Herbicides and Control Herbicide-Resistant Weeds
Open Access
- Author:
- Summers, Haleigh
- Graduate Program:
- Agronomy
- Degree:
- Master of Science
- Document Type:
- Master Thesis
- Date of Defense:
- December 17, 2019
- Committee Members:
- Heather D Karsten, Thesis Advisor/Co-Advisor
John M Wallace, Committee Member
Claudia Schmidt, Committee Member
Erin L Connolly, Program Head/Chair - Keywords:
- weed
integrated weed management
pigweed
horseweed
marestail
herbicide
glyphosate
dairy
cropping systems
cover crops - Abstract:
- Palmer Amaranth (Amaranthus Palmeri S. Watson), waterhemp (Amaranthus tuberculatus Moq. Sauer), and horseweed (Erigeron canadensis L.) are frequently the most troublesome herbicide-resistant weeds for farmers due to their rapid spread and presence of multiple resistance in some biotypes. Integrating multiple weed control strategies could be the solution to manage these weeds and stop the spread of resistance. Nine years of data investigating integrated weed management techniques on a no-till dairy farm provided some answers for weed control in Pennsylvania. We studied the impact of reducing herbicides and integrating other forms of weed management on both weed biomass and crop yields. Herbicides were reduced by banding herbicide over the crop row, substituting herbicides with mechanical weed control, and planting a companion crop with our perennial forage crop. We found that reduced herbicide typically led to increased weed biomass, however, crop yield was usually not affected. In the first year of perennial forage, weed biomass was determined by the annual nurse crop used and the environmental effects that year. We did not see an increase in weed biomass in the later years of the experiment as expected. Weeds did begin to increase in the reduced herbicide annuals but returned to low levels like standard herbicide by the second year of forage. Further research conducted over two years looked specifically at control of herbicide-resistant horseweed with an emphasis on new herbicide-tolerant soybean varieties and their corresponding herbicides, and how those weed control programs compared to using cultural or mechanical strategies to control winter-annual horseweed and other weeds. We found that mechanical control strategies performed similarly to using diverse modes of herbicide action at burndown and post-herbicide applications, providing another non-chemical management strategy that reduces selection pressure for herbicide-resistance. While results were not consistent between years due to wildlife damage to plots in the second year, we did find a trend that bolting plants taller than 10 cm were less likely to be controlled, confirming most herbicide labels. Finally, it was clear that using glyphosate alone is no longer an effective management strategy. In our third experiment, we studied cultural control methods for controlling herbicide-resistant summer-annual pigweed species by using cover crops, rotating to a frequently harvested perennial forage crop, delaying cover crop termination, or using a nurse crop. Delayed cover crop termination before soybean increased rye cover crop biomass, reduced pigweed density at two of three time points, and reduced end-of-season pigweed biomass better than early-termination or no cover crop, while also controlling total weed biomass better than early-termination of the cover crop. In alfalfa, oats cover crop did not winterkill, causing the cover crop to grow more than expected and into the spring-seeded alfalfa that was planted without any herbicides. Plots that contained oats had higher yield in the first harvest, and less pigweed biomass than pure alfalfa without a cover crop, with the oats cover crops providing the best total weed suppression. Results from all three studies show the benefits of using a diverse approach with multiple control tactics. We have found success with multiple tactics and recommend farmers start using IWM techniques, particularly including shallow tillage, increasing cover crop biomass, and rotating to a perennial forage.